The 'ultradeep' diamond, which weighs less than one-tenth of a gram, was found on the surface a decade ago by a Brazilian prospector. But it was formed in a region of the mantle known as the transition zone, some 250 to 400 miles below the surface. By contrast, most diamonds are formed at depths of about 100 miles.

NPR's Nell Greenfieldboyce reports that the find points to the likelihood of "huge amounts of water — about the same amount that's in all the world's oceans," deep underground.

"What researchers have found is a microscopic fragment of a mineral called ringwoodite. It's inside a diamond that was blasted out of the deep Earth by a prehistoric volcanic eruption," she says.

But, it's not liquid water, explains Graham Pearson, a geologist who studied the stone at the University of Alberta and is the lead author on the Nature study.

"What was most striking about the ringwoodite discovery ... was that 1.5 per cent of it was water, bound chemically to the mineral. Based on projections of how much ringwoodite scientists believe is in the Transition Zone, between 410 and 670 kilometres down, Dr. Pearson estimated that it contains 'a very, very large amount of water."

Although ringwoodite is thought to be one of the most common minerals in the Earth's interior, until now, it had only been seen in meteorites or made in the lab.

"Certain minerals have crystal structures that can form only at high pressures or temperatures, or both, and many rearrange themselves into different structures when the pressure is taken off or the temperature goes down. Thus, when the churning of the mantle brings rock towards the surface, some of the minerals that formed at great depths can no longer be found. But if the minerals are trapped inside diamonds, they stay compressed in their original forms."

"These high-pressure diamonds give you a window into the deep Earth,' says Pearson."